1. Technical Field
The present invention relates to phase retardation films and to liquid crystal display devices including the phase retardation films.
2. Description of the Related Art
In late years, applications of liquid crystal display devices (LCD) as information display devices, such as mobile phones, personal digital assistants (PDA), personal computers and television monitors, have been increasing rapidly due to their various advantages such as low power consumption, low voltage operation, light weight and reduced thickness. With development of the LCD technology, various modes of LCD have been proposed. Such proposals are solving problems with the LCD regarding response speed, contrast and viewing angle. However, the LCD are still pointed out that their viewing angle is narrower than that of cathode ray tubes (CRT), in other words, the LCD are of greater viewing angle dependency. Therefore, a variety of solutions for viewing angle compensation have been proposed.
As one of measures of viewing angle compensation, liquid crystal cells inherently capable of enlarging viewing angles have been proposed, for example, those of optically compensated bend (OCB) mode, vertical alignment (VA) mode and in-plane switching (IPS) mode.
Among them, the IPS mode is superior in viewing angle characteristic to the other modes because liquid crystal molecules orient parallel to substrate planes and in the same direction. However, viewing angle dependency is still a problem with various types of liquid crystal display devices with improved viewing angle characteristics including the IPS mode.
Various approaches have been proposed to compensate viewing angle dependency. One of them is an approach to compensate the viewing angle of liquid crystal display device by use of a phase retardation film. For example, Japanese Published Patent Application JP 11-133408 (corresponding to U.S. Pat. No. 6,115,095) discloses that a phase retardation film (a compensation layer) having an optic axis at a positive uniaxiality in a direction perpendicular to the substrate, namely, a phase retardation film in which molecules are oriented uniaxially in the thickness direction of the film is disposed between a liquid crystal substrate and a polarizing plate in the IPS mode.
However, viewing angle characteristics of liquid crystal display devices having a phase retardation film with an optic axis at a positive uniaxiality in a direction perpendicular to the substrate are still far inferior to those of CRTs.
As an approach to improve the viewing angle dependency of liquid crystal display devices of IPS mode, Japanese Published Patent Application JP 10-54982 (corresponding to U.S. Pat. No. 6,184,957) discloses to dispose, between a liquid crystal cell substrate and at least one polarizing plate, a monolayer phase retardation film (optical compensation sheet) made of a styrene-based polymer, which exhibits a negative uniaxiality when being uniaxially stretched.
Phase retardation films usually laminated with polarizing plates, liquid crystal cell substrates or the like through adhesive. However, monolayer phase retardation films composed of a styrene based polymer may crack due to temperature change when being laminated with other components such as polarizing plates and glass cells through adhesive and installed into liquid crystal display devices. The larger liquid crystal display devices, the more frequently cracking of phase retardation films occurs. In particular, it occurs very frequently in liquid crystal display devices of a size 30 inches or greater.
In addition, phase retardation films are required to have good reworkability. The reworkability is a performance which is required in the production of liquid crystal display devices. In the production of liquid crystal display devices, a phase retardation film is laminated first on a polarizing plate by a first adhesive. Then, a liquid crystal cell is laminated on the phase retardation film of the resulting laminate by a second adhesive. In the step of lamination to the liquid crystal cell, the laminate of the polarizing plate and the phase retardation film must, but often fails to, be combined with the liquid crystal cell at a high accuracy of angle adjustment. If the laminate and the liquid crystal cell are combined together at a wrong angle, the liquid crystal cell must be peeled away from the laminate and then combined again with another laminate of a polarizing plate and a phase retardation film. In the peeling of a phase retardation film from a liquid crystal cell, it is required that no chip of the phase retardation film or of the second adhesive used for laminating the phase retardation film to the liquid crystal cell remain on the surface of the liquid crystal cell. If a phase retardation film can be peeled off from a liquid crystal cell completely without leaving anything on the surface of the liquid crystal cell, the phase retardation film is considered to “be of good reworkability”. Monolayer phase retardation films made of a styrene-based polymer are of insufficient reworkability.